Method of and apparatus for the production of boron hydrides



May 15, 1951 s. D. LESESNE METHOD OF AND APPARATUS FOR THE PRODUCTION OFBORON HYDRIDES Filed Feb. 3, 1945 MNFJMMRNU WU@ I mventor Sewm/zB26365226 B MJL/a my ttorneg Patented May 15, 1951 UNITED STATES PATENTOFFICE METHOD OF AND APPARATUS FOR THE PRODUCTION OF BORON HYDRIDESSherman D. Lesesne, Worcester, Mass., assignor,

by mesne assignments, to Allied Process Corporation, New York, N. Y., acorporation of New York Application February 3, 1945, Serial No. 576130This invention relates to a method of and apparatus for the productionof compounds of boron and hydrogen, and it has particular reference tothe production, in substantial quantities, of the simplest of suchcompoundsdiborane The group of compounds broadly encompassed by the termboron hydrides comprises a number whose molecule is composed of atoms ofboron and hydrogen, together with various addition products orderivatives thereof. They have heretofore been primarily of theoreticalinterest, and have been prepared only with great difficulty. Theliterature of the subject is nicely summarized in two publications, towhich, with their bibliographies, the reader is referred for a survey ofthe art as it has heretofore existed. 'I'hese are: (l) Hydrides of boronand silicon, by Alfred Stock, Cornell University Press, 1933, and (2)Recent developments in the chemistry of the boron hydrides, by H. I.Schlesinger and Anton B. Burg, 31 Chemical Reviews 1, August ture, itmay be noted that there are known two general types of substances havinga boron-hydrogen molecule, respectively called the boranes and thehydroboranes To them are respectively ascribed empirical formulae suchas BHI-IULM), and Bnl-lows), the boranes being relatively more stable.There also appear to be other compounds having a differentboron-hydrogen ratio, and which have not as yet been fully identied. Ofthe boranes, the first of the series is diborane, B21-Is, which is acolorless gas at ordinary conditions of temperature and pressure, butwhich may be solidified at about -165.5 C., and which has a boilingpoint of -92.5 C. By suitable treatment, diborane may be converted intoother boron-hydrogen compounds or derivatives, and it thus may beregarded as a basic material for purposes of synthesis.

Due to the richness of diborane in hydrogen, it is moreover a usefulsource of elemental hydrogen, of especial value when it is desired tocondense the available hydrogen content into a smaller space than wouldbe occupied by an equivalent content of hydrogen itself. Thus, whendiborane is hydrolyzed by water and acid, six volumes of hydrogen areliberated according to the following equation:

Heretofore, however, no method or means has been available, despite theintensity of scientific study, to provide the boron hydrides inquantitiesv Without attempting here to abstract the litera y 2 Claims.(Cl. 23-204) of more than a few grams at a time. Stocks synthesis waspredicated on the decomposition of magnesium boride with an aqueous acidsolution, which precluded the direct formation of diborane due to thehydrolytic effect just noted, out which yielded small quantities ofcrude gas which had to be further processed. In first describing animproved method, Schlesinger and Burg reported.r that with the Stockmethod, a month of tedious work is required vfor the production of a fewgrams of a mixture of several hydrides. 53 JACS 4321 (1931) Hydrides ofBoron. I. An Efficient New Method of Preparing Diborane.) These laterinvestigators devised a method of treating a suitable substance, such asgaseous boron trichloride or boron bromide, with hydrogen in theelectric arc at low pressure. They reported that their method produced asmall amount of diborane directly, together with halogenated diboranewhich could be further treated to yield, additional quantities ofdiborane; and thatthereby they could produce from ive to ten grams perweek.

By contrast with the results heretofore obtained, the present inventionenables one to produce diborane in pure form, and quantitatively, inamounts limited only by the raw,materials and equipment available. Thepresent inven-l tion thus permits the industrial preparation ofdiborane, and other boron-hydrogen compounds, for the first time.

The present invention involves a reaction between a salt-like or salinehydride of a metal and a boron halide, of which the following is atypical equation:

I have discovered that a compound such as lithium hydride, particularlywhen pulverized to have a large available surface area, and a compoundsuch as boron fluoride, will react under suitable conditions either in adry wav, or in the presence of a non-aqueous suspending or dispersingliquid, in accordance with the foregoing equation. When a liquid isused, it should not, if good eilicienciesare to be expected, bedestructively reactive with either the hydride or the halide, or thedesired end product, ncr should such liquid otherwise inhibit thedesired reaction. Examples of suitable liquids will be givenhereinafter.

The class of substances denominated the saltlike or saline hydrides aresuccinctly described in Ephraim, InorganicChemistry, (Thorne & Roberts4th edition, pp. 87'7, 878, Nordeman Publishing Co., New York) ascomprising hydrides of the alkalis and the alkaline earths, with certainexceptions. In them, according to theory, the hydrogen acts as anegative ion, rather than as a positive ion as is generally consideredin many 5 reactions, Ofv thgll'ipus hydrides: and halides, which havebeen available for study in connection With the present invention, thelithium hydride and boron uoride combination has been' found to be mosteffective, and,accordinglyfsuch.- combination is regarded as the bestnow knownv for the ecient practice of the invention.V

This conclusion is based ontheV following-observations, which also showthereactivity of-'other hydrides and halides. When boron fluoride is l5replaced with boron bromide, some hydrobromic acid is also formed, thusdetracting fromtheef.- fciency, and also contaminating the diborane..When calcium hydride and boron fluoride are used, the yield of, diboraneis positive,.but the reactionA proceeds much mgre slowly,`and c lo'eslnot appear to go to completion.Y When lithium hydrideand boron chlorideare used, diborane isformed, b ut only in small amounts underthe sameconditions which enable'speciflc reaction (2) to go to completion. Whensodium hydride'` is treatedwith boron chloride, a reaction' takesvplace, with the formation of boron-hydrogen4 molecules thatv appear tohave a formulariother than Bcl-M Underthe same conditions/of temperatureand pressure, however, lithium liydridew and boron, fiuoride may be madeto react with much higher yields, and accordingly these twoVVVsubstances maybe regarded as most efficacious.,

In conductingthe process in either'a wet or a dry way, thehydrideshould-be pulver-ized, as by` grinding until all the particles pass aneighty mesh sieve. When grinding, moistureor-atmosf pheric air shouldbeY erludedas'much as possible, because the hydride may otherwisedecompose with the formation of hydroxidesV or carbonates, andthus-*detract from che overallreaction efficiency. The hydride powdermay-thenlceY contacted with the boron halide, and, undersuitabletemperature conditions, the boron-hydrogen compounds will then beformed.' Y

In-oneway of-carrying out thefprocess,there was set up thecustomarycombustion tube ap paratus, such as is usedforithedetermination` offcar-bon in steel;V Into the boat was placed some powdered lithiumhydride, and, after.- the tube wasA swept free of air by al stream ofnitrogen, gaseous boron'ha'lide (i. e., fluoride orn bromide) wasfpassed Yover the hydride, while heating the boat with Bunsen burners.The products ofthel .5 reaction included diborane; diluted. withunreacted halide, and' also White cruciform crystals, Qffgood'stablity,which were depgsiledainthefwld portion 0f the d`ss 1-1argetube,vSinceit-is lingua that diborane be decomposed-,bv heette-yield otherboron hyd-rides theprescmeof, auch Clt-ys: tails maybeaccountedjrfoigonthis basis: While? such procedure.illustratesonecway of conducti f thereaction,A better resultsA are. obtainable. by usine.- Oher, types, O-apparatus, and; Somewhat@ n vdifferent technique; eshereinafterdesribedi,

4 reaction zone, and, by placing a reflux condenser in the dischargeline, the liquid may be retained, while the relatively pure diboranepasses forward to a point of collection. Various advantages of thisprocedure will be made more apparent as this description proceeds;

A- further discussion of-the princi-ples ofthe invention, and thevarious features thereof, will be made with reference to theaccompanying drawings*showingl suitable apparatus and where- 1n:

Fig. 1 is an elevation of equipment suitable for conductingtheprocess inthe wet way; and

Figf Zjisanlelevation of part of the apparatus which may be used` inconducting the process in a dry way.

In Fig.- 1', a,l reaction flask I0 is provided with three necks, II, I2,and I3, respectively receiving a dropping tube I4, a stirrer I5protected by a mercury seal I6, and the lower end of a reflux condenserI'I. One'of the reagents, for. example the powdered hydride, isplaced-Withsome liquid4 in the flask I0 and is maintained ina suspended,orrdispersed condition by the stirren I5. Theother.r reagent, such, forexample, as theboron halide, is then admittedvat a suitablerate fromthedropping tube I4. VolatileA reaction products, pass. into theWater-cooled condenser, which is of--the low temperature type. That: is,its uppery end; is` enlarged to receive a thimbleY I8 containing.` a,mixture of solid carbonl dioxideorffdry. ice. and chloroform, so thathigh boilingA point substances.; (as compared to th-e diborane)`arecondensedifor. return to the flask l 0.

A discharge line I 9 communicates 4with the Lupe. per end of the thimbleI8,- and it leads to a liquidi trap 2 I` maintained at a temperature,close to` 78 C; byV a surroundingliask 22 packed with Dry Ice. Tosafeguard againstthe possibility. that desired material, will passthereux condenser- I 1,- this trap is useful to separateingredintscondense.. ing above the trap temperature. In the hereinafter:`described exam-ples, the amountV ofv condensate in the trap 2I wasfound'to be .quite limited.

The gas not condensed in thetrap. 2I passes,

through a branch line 2,3, to Whichisiconnected a. manometer 25 fordetermining` pressures, anda-` reaction tube. 25Vwhich may beheated; asbythe,` Bunsen burner-26.V It has already-beennotedthat the .wet methodmay.r beY operated-1 to lproduce hightyields-of diborane, which may,if.y desired, beide:k composed by heat into othenboronv h-yidrides.y Thereactiontube 2.5` thereforeillustrates-a way., inwhich the end` productmayv belmodiedafrom; the. products. initially,y formed in the flask-III.;` although for many purposes thispart ofrthe ape. paratus will notbe needed. InsuchI event, no. heat isY supplied to the tubeY 2,5, whichIthenisf; simplyv a portion of,l thev conduit` through which. thediborane passes. The reaction tube-,215 comr.. municates with a line.2lhaving a branch 28i connected to a vacuumpump-Knot shown), so

that the whole systemY maybe exhausted: asgdef.v

sired, and a branch 29fconnected toa collecting; tube 3I which is cooledbyallask 327 containing` liquid nitrogen. Such, cooling mediurn-producesa temperature of about 196f0., and ifhereloreVV effects the solidicationofdiborane.

The remaining parts of-gfthe, apparatuswill be recognized as simpleequipment for Conductingtests and analyses.v Thereis, provided the gasburette 35, connected to the collecting tube 3I by a line. 36, and'whosevolumemay beregulated by mercury containedfinYtheattachedIseparatoryfunnel 3l,y togetherwith a fhydroxlsis., chambers38, measuring tube 39, and water admission funnel 4 I. As these devicesrelate to analytical methods employed to determine the efficiency of theinvention, and as their nature is apparent to those skilled in the art,it is not believed a further description is required. The drawingdiscloses how they may be associated with the previously described partsof the apparatus. It will likewise be obvious that the variousstopcocks' illustrated may be operated as conditions require, and thatthe reaction flasks, condensers, and other parts may be duplicated toincrease the capacity. Chemical glass may be used in preparing thisequipment, with the stopcocks well fitted. It may be of interest toreport that while this apparatus is quite simple, and of ordinarylaboratory type, good results have been obtained with it and without thediiculties explained by Professor Stock (loc. cit. p. 173) In Fig. 2,the reaction flask I9 has been replaced by a structure which essentiallyis a ball mill A glass jar 5| of about one liter capacity is secured toa chuck 52 which is driven for rotation at about 60 R. P. M. through asmall motor and speed reduction gear set 53. The mouth of the jar isground to receive a non-rotating glass tube 5d provided with a smallerinner tube 55 leading to the back of the jar, and connected at its outerend to a line 56 and a second line 5l'. It may be noted here that theline 56 is used to sweep out the apparatus with a current of drynitrogen before instituting the reactions, the presence of oxygen ormoisture obviously being detrimental. Products of the reaction passthrough the mouth of the jar through the large tube 54 into a line 59,and thence into a flask containing a liquid such as ether, and which inturn is connected to the reflux condenser Il and the other apparatus asheretofore described.

In operation, the jar 5| is charged with flint balls of about two tothree centimeters in diameter, and some finely divided hydride. Heat issupplied through a Bunsen burner positioned below the jar, and the boronhalide is led in through the line 5l. As the jar rotates, the ballsbreakup the powder and prevent the formation of solid masses, and thuspresent fresh surfaces of the hydride to the entering halide. In thework in which this equipment was used, solid boron hydrides weredetected in the relatively cold stretches of the f large tube 54 and theline 58, while the interposed flask 59 of ether provided a means fortrapping or dissolving substantial quantities of uncon- Verted boronhalide. By providing the usual elecl tric light bulb, the remainingboron hydrides are easily distilled into the reflux condenser forfurther separaticn and collection.

In one run with the apparatus shown in Fig. 2, the jar 5| was chargedwith a quantity of lithium hydride which had been ground to pass aneighty mesh screen. The apparatus was then swept out with nitrogen fromthe line 56, and gaseous boron fluoride was thereafter admitted throughthe line 51. The reaction initially proceeded at a slow rate, but muchmore rapidly as the jar 5I was heated to a temperature between 150 and250 C. The rotation of the jar and the action of the hint Iballs on theparticles of hydride served to breakup the mass and to present freshsurfaces to the action of the gas, thus enabling the reaction toproceed. While such reaction is exothermic, the radiation losses fromthe simple equipment employed were such that some make-up heat wasrequired during the run.

The gases flowing through the discharge tube 54 Awere bubbled throughboiling anhydrous ethyl ether in the flask 59, wherein a substantialportion of the unconverted boron fluoride was dissolved. The distillatefrom the flask was then passed through the reflux condenser I'I, toreturn the ether and other materials condensing below the temperature ofthe thimble I8, and to forward the residue to the collecting tube 3|.With intermittent operation, 16.5 grams of impure diborane were obtainedafter two days. The nal condensate contained some BFS, which may havecome over as B2H5F and subsequently decomposed, and diborane, asdetermined by the hydrolysis reaction. By fractional condensation, thepurity was successively increased to 77%, 91%, and 95%.

It will be apparent, from this example, that the invention provides forthe production of diborane in much greater quantities, and with yieldsof higher initial purity, than attainable heretofore. The reaction maybe conducted either intermittently or continuously, and it will beobvious that even better results may be obtained according to the dryprocedure by suitable reiinements in the apparatus.

Referring again to Fig. l., the following example will illustrate howthe invention may be practiced in the wet way. The ask I8 was chargedwith 1600 cc. of anhydrous ethyl ether and 116.7 grams of pulverizedlithium hydride having an LiH content of '77%. The amount of availablelithium hydride was therefore equivalent to 89.9 grams. The particleswere dispersed through the ether by means of the stirrer I5. Thedropping tube I4 was charged with a saturated solution of boron fiuoridein ether, which was prepared by bubbling the gas into the ether atapproximately 0 C. until absorption ceased, and thereafter fractionatingbetween 123 C. and 126 C. The liquid so obtained is an addition compoundof ether and boron fluoride, as represented by the formula (C2H5)2O:BF3, and which, under the conditions of operation, is so unstable asto release the halide readily in the reaction zone. The contents of thetube I4 were moreover saturated with diborane prepared from a previousoperation of the process.

Upon opening the stopcock of the dropping tube I4, the reactionproceeded at room temperature, increasing somewhat in rate andliberating enough heat to cause the liquid to boil evenly and thusdistill the reaction products into the reux condenser I1. Thisuniformity of reaction may be attributed in part to the mass of theether present, and to the dissipation of the heat through the condenser,thus maintaining a substantially constant reaction temperature. Verylittle liquid or boron fluoride passed the cold thimble I8, and thissmall amount was caught by the trap 2|, thus permitting pure gas to goforward to the collecting tube 3| for condensation by liquid nitrogen inthe container 32.

At the end of the reaction, which was within twenty-four hours from thetime of starting, there was recovered 47.2 grams of pure diborane,B21-I6, or 90.2% of the theoretical maximum yield. The difference isaccounted for by losses in the apparatus and through the mercury sealI6, thus indicating that the reaction had proceeded quantitatively tocompletion. The end product showed, upon analysis, no trace of fluoridecompounds, and purities of 99.5% and 99.8% respectively by thehydrolysis test. Repetitions of antenas providesa means. of preparingdiborane. directly.

in. afpure state, and in quantities greatly in' ex cess of those..heretofore. obtainable;

Withrespectto the foregoing example,v it. may be'. pointed out that the.employment of ethyl. ether,v has a numberl of practical. advantages,although, asl previously noted, other liquids mayy be used. The etherdoes notenter thereaction. by undergoingv chemical decomposition, in.fact, it.. has-a high degree of stability under the describedvconditions of operation.v Its. relatively low boiling`4 point (about 35C.). permitsfthe. rapid.l removal of the gaseous.V boron hydridesA at alow temperature, whilethe, comparativelyA high'. boilingpoint of the`ether-boron fluoride complex (about 126 0.-.) tends to keep.anyunreactedhalide from passing. the. reflux condenser.;

As previously explained, etherisa solvent. for.

the boron halide, forming a solutionoraddi.- tioncompoundw-hich isunstable inthe reaction Zone, but it is not a good solvent.: atitsrboilingf.

temperature, for the diborane. These-attributes simplify the conduct ofthe reaction, enabling itvto'proceeduniformly;r in simple apparatus,

and with normal external conditions of atmos.

phericf temperature and pressure. 'I-'he separa.-

tion of the diborane from the reactants in situ;`

as: described in connectionwith the action fin thecondenser'l'l,moreover leads to the direct recovery.v ofl a pure end product.,

The.4 additionv of some diboraney to lthe reaction'.-

cedure-just outlined may be modified invarious-A ways.l Thus, a slurryof the hydrid'e 'in a. suit-f able carrier liquid may'be formed-outside`off the reaction vessel and supplied to it concur-` rently with. orafter the introc'luction of gaseous or dissolved boron halide. Forcontinuousoperation, the lithium fluoride. formed as a reac-V tionproduct may also be withdrawn from. time to time, so as to make way forfresh quantities of the reactants. The .liberated diborane may becollected as described in a solid state, or it may be collected as a gasorv taken. to some other collection point for conversion to some otherproduct.

From the foregoing` description, it will be seen that. the presentinvention provides a novel and. effective means for the production. ofthe boron hydrides in large quantities, with simple apparatus, and. withyields andv eiliciencies greatly exceeding those heretofore attainable.In View of the` high efficiency obtainable with the wet process, itwouldv appear that this way' of' con:V ducting the invention representsthe procedure now best known. It has, however, beenmade clear that theinvention may be practiced ,in various ways, and it isof courserecognized that the problems of industrial and engineering` chemistrymay often dictate a. sacrice of. eiciency, as such, to achieve someother purpose'. Hence,l it is not intended to limit the inventionY toany specific example given, but to encompass' all such Variations andmodifications as fall'I within the scope ofthe following claims.

l. In Vthe preparation of diborane, the steps which comprise bringingtogether inely` divided lithium hydride and boron fluoride in thepresence of non-aqueous liquid, and adding theretol some diborane toexpedite the'reaction.

2. The method of preparing` diborane which comprises continuouslycontacting a suspension of powdered alkali metall hydride in an inertliquid containing added diborane with boron triuoride and condensing'the' diborane produced.

SHERMANV D; LESESNE.

No referencescited.

1. IN THE PREPARATION OF DIBORANE, THE STEPS WHICH COMPRISES BRINGINGTOGETHER FINELY DIVIDED LITHIUM HYDRIDE AND BORON FLUORIDE IN THE PRES-